There is a major unresolved controversy of whether non-neutralizing antibodies (nnAbs) with potent Fc-receptor mediated (FcR)-effector functions can block HIV/SHIV acquisition. Accordingly, the long-term goal of our program is to test the hypothesis that vaccine-elicited nnAbs with potent FcR-effector functions and directed at epitopes in the C1/C2- and V2- regions of gp120 protect against SHIV acquisition. This hypothesis will be tested in two steps. First, through aims proposed with this application, we will identify an optimal immunogen/adjuvant formulation to elicit these responses in small animals. Second, through a small proof of concept study, we will evaluate the immunogen/adjuvant formulation in a repeat, low-dose SHIV162P3 challenge model. The principal significance of this project is that it will either support or refute the above hypothesis, providing new information critical to HIV-1 vaccine development. Considerable evidence points toward a role of FcR-effector functions of Abs including antibody-dependent cellular cytotoxicity (ADCC) toward non-neutralizing epitopes in the C1 region of gp120 (A32-like epitopes) in preventing or modulating HIV-1 infection and in vaccine induced protection in humans. The latter is largely supported by results of the RV144 vaccine trial that implicated Ab responses to A32 sub-region with reduced infection risk in a subset of vaccines. Furthermore, Abs specific for C1 region and linear V2-epitopes synergized for infectious virus capture and ADCC, suggesting the cross-talk between these specificities contributing to vaccine efficacy due to FcR-effector functions. With this application we aim to develop inner domain-based immunogens (ID) capable of inducing solely the nnAbs directed at epitopes identified as targets of FcR- effector response in the RV144 trial. We propose these novel ID constructs, further optimized for selective presentation of ADCC epitopes and/or multimerized to develop into new immunogens effective in selective inducing FcR-effector Ab responses directed at one (A32 sub-region) and both (A32 sub-region and V2 loop) Env targets associated with protective ADCC responses in human. Our ID immunogen candidate consists of the inner domain of the gp120 core stabilized in CD4-bound conformation. ID stably presents A32-like epitopes within a minimal stable structural unit and is a platform for further structure based optimization and modification. Aim 1 develop monomeric and multimerized variants of ID and ID-V1V2, both expressing the ADCC epitopes. Aim 2 will evaluate antigenicity of monomeric and multimeric variants of ID and ID-V1V2 and Aim 3 will evaluate the immunogenicity of monomeric and multimerized variants of ID and ID-V1V2 in BALB/c mice. These studies will complete the first step in testing the hypothesis that vaccine-elicited nnAbs protect against SHIV acquisition; identification of an immunogen. The revised work scope does not provide sufficient time to carry out a SHIV162P3 challenge study; however, once a suitable immunogen formulation is in hand, institute funds will be provided for a preliminary study while funding is sought for project continuation.